"inside diameter of a gun barrel," 1580s, from French calibre (by mid-16c., perhaps late 15c.), often said to be ultimately from Arabic qalib "a mold for casting." Barnhart remarks that Spanish calibre, Italian calibro "appear too late to act as intermediate forms" between the Arabic word and the French.
But English Words of Arabic Ancestry finds that the idea of an Arabic source "comes with no evidence and no background historical context to support it. It is far more likely that the word was formed in French" from Medieval Latin qua libra "of what weight" (a theory first published 19c. by Mahn), from fem. ablative of quis (from PIE root *kwo-, stem of relative and interrogative pronouns) + ablative of libra "balance" (see Libra).
In U.S., expressed in decimal parts of an inch (.44-caliber = ".44-inch caliber"). The earliest sense in English is a figurative one, "degree of merit or importance" (1560s), from French. Later, figuratively, "the capacity of one's mind, one's intellectual endowments."
also callisthenics, kind of light gymnastics, 1842, (the adjective calisthenic/callisthenic, of exercises, was in use by 1837), formed on model of French callisthenie, from Latinized combining form of Greek kallos "beauty" (see Callisto) + sthenos "strength, power, ability, might" (perhaps from PIE root *segh- "to have, hold," on the notion of "steadfastness, toughness") + -ics.
"Of this at least I am certain, that none but a born romp and hoyden, or a girl accustomed [to] those new-fangled pulleyhauley exercises, the Calisthenics, is fitted for the boisterous evolutions of a sea-voyage." [Thomas Hood, "The Schoolmistress Abroad," New Monthly Magazine, 1842]
Originally, gymnastic exercises suitable for girls and meant to develop the figure and promote graceful movement. OED describes the word as "chiefly a term of young ladies' boarding-schools." A place for doing it was a calisthenium (1853). The proper Greek, if there was such a word in Greek, would have been *kallistheneia.
mid-13c., "to cry out; call for, summon, invoke; ask for, demand, order; give a name to, apply by way of designation," from Old Norse kalla "to cry loudly, summon in a loud voice; name, call by name," from Proto-Germanic *kall- (source also of Middle Dutch kallen "to speak, say, tell," Dutch kallen "to talk, chatter," Old High German kallon "to speak loudly, call"), from PIE root *gal- "to call, shout." Related: Called; calling.
Old English cognate ceallian "to shout, utter in a loud voice" was rare, the usual word being clipian (source of Middle English clepe, yclept). Old English also had hropan hruofan, cognate of German rufen. Coin-toss sense is from 1801; card-playing sense "demand that the hands be shown" is from 1670s; poker sense "match or raise a bet" is by 1889. Meaning "to make a short stop or visit" (Middle English) was literally "to stand at the door and call." Telephone sense is from 1882.
To call for "demand, require" is from 1530s (earlier in this sense was call after, c. 1400). To call (something) back "revoke" is from 1550s. To call (something) off "cancel" is by 1888; earlier call off meant "summon away, divert" (1630s). To call (someone) names is from 1590s. To call out someone to fight (1823) corresponds to French provoquer. To call it a night "go to bed" is from 1919.