ballpark (n.)

"round object, compact spherical body," also "a ball used in a game," c. 1200, probably from an unrecorded Old English *beal, *beall (evidenced by the diminutive bealluc "testicle"), or from cognate Old Norse bollr "ball," from Proto-Germanic *balluz (source also of Dutch bal, Flemish bal, Old High German ballo, German Ball), from PIE root *bhel- (2) "to blow, swell."

The meaning "testicle" is from early 14c. (compare ballocks). Ball of the foot is from mid-14c. The meaning "rounded missile used in warfare" is from late 14c.

The meaning "a game played with a ball" is from mid-14c. The baseball sense of "pitch that does not cross the plate within the strike zone" is by 1889, probably short for high ball, low ball, etc.

Ball-point pen is by 1946. Ball of fire when first recorded in 1821 referred to "a glass of brandy;" as "spectacularly successful striver" it is attested from c. 1900. Many phrases are from sports: To have the ball "hold the advantage" is from c. 1400. To be on the ball is from 1912; to keep (one's) eye on the ball in the figurative sense is by 1907, probably ultimately on golf, where it was oft-repeated advice. Figurative use of ball in (someone's) court is by 1956, from tennis.

The head must necessarily be steady, for it is most important that you should keep your eye fixedly on the ball from the moment that the club-head is lifted from the ground until the ball is actually struck. "Keep your eye on the ball," should be your companion text to "Slow back." [Horace G. Hutchinson, "Hints on the Game of Golf," 1886]

Once a meeting is over, someone will be expected to do something. Make sure it is someone else. This is known as keeping the ball in their court. [Shepherd Mead, "How to Get Rich in TV Without Really Trying," 1956]

mid-13c., "tract of land enclosed as a preserve for beasts of the chase," from Old French parc "enclosed wood or heath land used as a game preserve" (12c.), probably ultimately from West Germanic *parruk "enclosed tract of land" (source also of Old English pearruc, root of paddock (n.2), Old High German pfarrih "fencing about, enclosure," German pferch "fold for sheep," Dutch park).

Internal evidence suggests the West Germanic word is pre-4c. and originally meant the fencing, not the place enclosed. It is found also in Medieval Latin as parricus "enclosure, park" (8c.), which likely is the direct source of the Old French word, as well as Italian parco, Spanish parque, etc. Some claim the Medieval Latin word as the source of the West Germanic, but the reverse seems more likely. Some later senses in English represent later borrowings from French. OED discounts the notion of a Celtic origin: Welsh parc, Gaelic pairc are from English.

Meaning "enclosed lot in or near a town, set aside and maintained for public recreation" is attested from 1660s, originally in reference to London; the sense evolution is via royal parks in the original, hunting sense being overrun by the growth of London and being opened to the public. It was applied to sporting fields in American English from 1867.

New York's Park Avenue as an adjective meaning "luxurious and fashionable" (1956) was preceded in the same sense by London's Park Lane (1880). As a surname, Parker "keeper of a park" is attested in English from mid-12c. As a vehicle transmission gear, park (n.) is attested from 1949.