Old English leornian "to get knowledge, be cultivated; study, read, think about," from Proto-Germanic *lisnojanan (cognates: Old Frisian lernia, Middle Dutch leeren, Dutch leren, Old High German lernen, German lernen "to learn," Gothic lais "I know"), with a base sense of "to follow or find the track," from PIE root *lois- "furrow, track." It is related to German Gleis "track," and to Old English læst "sole of the foot" (see last (n.1)).

From c. 1200 as "to hear of, ascertain." Transitive use (He learned me (how) to read), now considered vulgar (except in reflexive expressions, I learn English), was acceptable from c. 1200 until early 19c. It is preserved in past-participle adjective learned "having knowledge gained by study." Old English also had læran "to teach" (see lere). Related: Learning.

Old English lar "learning, what is taught, knowledge, science, doctrine; art or act of teaching," from Proto-Germanic *laisti- (compare Old Saxon lera, Old Frisian lare, Middle Dutch lere, Dutch leer, Old High German lera, German Lehre "teaching, precept, doctrine"), from PIE root *lois- "furrow, track;" compare learn.

late Old English tecunge "act of providing guidance or training to another, imparting of instruction or knowledge," verbal noun from the source of teach (v.). Gradually passing into modern sense "business of instructing." As "that which is taught, knowledge or understanding imparted," it is attested from c. 1300. Middle English also had teachingless (adj.) "deprived of instruction, untaught" (mid-14c.).

Old English tacen "sign, symbol, evidence, portent" (related to verb tæcan "show, explain, teach"), from Proto-Germanic *taikna- (source also of Old Saxon tekan, Old Norse teikn "zodiac sign, omen, token," Old Frisian tekan, Middle Dutch teken, Dutch teken, Old High German zeihhan, German zeichen, Gothic taikn "sign, token"). According to Watkins, this is from PIE root *deik- "to show," also "pronounce solemnly." Compare, from the same root, German zeigen "to show," Old English teon "to accuse,"

Also in late Old English as "observable characteristic or action indicating an inner state; means of identifying a person." By c. 1200 of physical objects representing an action, state, deity, etc.; "keepsake, that which serves as a reminder." From late 14c. as "act performed in recognition of a contract or agreement; public, symbolic display or act." The meaning "coin-like piece of stamped metal" is by 1590s.

The Middle English sense of "evidence, support for a belief" is retained in by the same token (mid-15c.), a phrase used in introducing a corroborative circumstance, according to Century Dictionary "almost equivalent to 'this in testimony.' "

Middle English bitechen, from Old English betæcan "give up to, impart, deliver; appoint, set apart, dedicate," from be- + teach (v.). The form and sense have been confused with betake. The meaning "impart, teach" is from c. 1300. The word was obsolete or archaic from 16c. Related: Betaught; beteaching.

also re-teach, "to teach over, teach again or anew, supply with new teachings," 1640s, from re- "back, again, anew" + teach (v.). Related: Retaught; reteaching.

past tense of teach (v.), from Old English tahte, past tense of tæcan. As an adjective, of a person, "instructed, trained," by late 14c. The 1382 Wycliffe Bible has taughtly (adv.) "with craft, skillfully."

For the unrelated adjective meaning "stretched or pulled tight" (which formerly was also spelled taught), see taut.

mid-15c., techeable, of a fact or idea, "capable of being taught," from teach (v.) + -able. In reference to persons, "able to receive instruction," late 15c., also "able to teach" (a sense now obsolete).

In reference to subjects, "appropriate for instruction," from 1660s. Related: Teachably; teachableness; teachability. An Old English word for it was leorningende. Teachable moment, attested from 1917, is not common until after c. 1960. 

mid-14c., techer, "one who provides moral guidance to another;" late 14c., "one who gives instruction in a field or craft;" agent noun from teach (v.).

It was used earlier in a sense of "index finger" (early 14c.). By c. 1400 as "animal trainer" Fem. form teacheress is attested from late 14c. (in reference to Wisdom, translating Latin doctrix). Teacher's pet "student aspiring to be favored by a teacher" is attested from 1856; teacher's aide is by 1956. Related: Teacherly.

Proto-Indo-European root meaning "to show," also "pronounce solemnly," "also in derivatives referring to the directing of words or objects" [Watkins].

It forms all or part of: abdicate; abdication; addict; adjudge; apodictic; avenge; benediction; betoken; condition; contradict; contradiction; dedicate; deictic; deixis; dictate; diction; dictionary; dictum; digit; disk; ditto; ditty; edict; Eurydice; index; indicate; indication; indict; indiction; indictive; indite; interdict; judge; judicial; juridical; jurisdiction; malediction; malison; paradigm; policy (n.2) "written insurance agreement;" preach; predicament; predicate; predict; prejudice; revenge; soi-disant; syndic; teach; tetchy; theodicy; toe; token; valediction; vendetta; verdict; veridical; vindicate; vindication; voir dire.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dic- "point out, show;" Greek deiknynai "to show, to prove," dikē "custom, usage;" Latin dicere "speak, tell, say," digitus "finger," Old High German zeigon, German zeigen "to show," Old English teon "to accuse," tæcan "to teach."