unwelcome (adj.)

Old English wilcuma "welcome!" exclamation of kindly greeting, from earlier wilcuma (n.) "welcome guest," literally "one whose coming suits another's will or wish," from willa "pleasure, desire, choice" (see will (n.)) + cuma "guest," related to cuman "to come," from PIE root *gwa- "to go, come." Similar formation in Old High German willicomo, Middle Dutch wellecome.

Meaning "entertainment or public reception as a greeting" is recorded from 1530. The adjective is from Old English wilcuma. You're welcome as a formulaic response to thank you is attested from 1907. Welcome mat is from 1908; welcome wagon is attested from 1940.

prefix of negation, Old English un-, from Proto-Germanic *un- (source also of Old Saxon, Old Frisian, Old High German, German un-, Gothic un-, Dutch on-), from PIE *n- (source of Sanskrit a-, an- "not," Greek a-, an-, Old Irish an-, Latin in-), combining form of PIE root *ne- "not." Often euphemistic (such as untruth for "lie").

The most prolific of English prefixes, freely and widely used in Old English, where it forms more than 1,000 compounds. It underwent a mass extinction in early Middle English, but emerged with renewed vigor 16c. to form compounds with native and imported words. It disputes with Latin-derived cognate in- (1) the right to form the negation of certain words (indigestable/undigestable, etc.), and though both might be deployed in cooperation to indicate shades of meaning (unfamous/infamous), typically they are not.

It also makes words from phrases (such as uncalled-for, c. 1600; undreamed-of, 1630s; uncome-at-able, 1690s; unputdownable, 1947, of a book; un-in-one-breath-utterable, Ben Jonson; etc., but the habit is not restricted to un-; such as put-up-able-with, 1812). As a prefix in telegramese to replace not and save the cost of a word, it is attested by 1936.