comma n.

c. 1300 (mid-12c. in surnames), "small axe with a short handle," designed to be used by one hand, from Old French hachete "small combat-axe, hatchet," diminutive of hache "axe, battle-axe, pickaxe," possibly from Frankish *happja or some other Germanic source, from Proto-Germanic *hapjo- (source also of Old High German happa "sickle, scythe").

This is perhaps from PIE root *kop- "to beat, strike" (source also of Greek kopis "knife," koptein "to strike, smite," komma "piece cut off;" Lithuanian kaplys "hatchet," kapti, kapiu "to hew, fell;" Old Church Slavonic skopiti "castrate," Russian kopat' "to hack, hew, dig;" Albanian kep "to hew").

Hatchet-face in reference to one with sharp and prominent features is from 1650s. In Middle English, hatch itself was used in a sense "battle-axe." In 14c., hang up (one's) hatchet meant "stop what one is doing." Phrase bury the hatchet "lay aside instruments of war, forget injuries and make peace" (1754) is from a Native American peacemaking custom described from 1680. Hatchet-man was originally California slang for "hired Chinese assassin" (1880), later extended figuratively to journalists who attacked the reputation of a public figure (1944).

"punctuation mark consisting of two dots, one above the other, used to mark grammatical discontinuity less than that indicated by a period," 1540s, from Latin colon "part of a verse or poem," from Greek kōlon "part of a verse," literally "limb, member" (especially the leg, but also of a tree limb), also, figuratively, "a clause of a sentence," a word of uncertain etymology.

The meaning evolved in modern languages from "independent clause" to the punctuation mark that sets it off. In ancient grammar a colon was one of the larger divisions of a sentence.

early 15c., periode, "a course or extent of time; a cycle of recurrence of a disease," from Old French periode (14c.) and directly from Medieval Latin periodus "recurring portion, cycle," from Latin periodus "a complete sentence," also "cycle of the Greek games," from Greek periodos "cycle, circuit, period of time," literally "a going around," from peri "around" (see peri-) + hodos "a going, traveling, journey; a way, path, road," a word of uncertain origin (see Exodus).

Sense of "repeated cycle of events" led to that of "interval of time." From 1712 as "an indefinite part of any continued state or series of events;" by 1727 as "time in which a circuit or revolution (as of a heavenly body) is made." Sense of "episode of menstruation" is by 1829, probably short for period of menstruation (1808), etc.  

The meaning "dot marking end of a sentence" is recorded c. 1600, from the earlier sense of "a complete sentence, from one full stop to another," then "a full pause at the end of a sentence" (1580s). The educational sense of "portion of time set apart for a lesson" is from 1876. The sporting sense "division of a game or contest" is attested by 1898. As an adjective from 1905; period piece is attested from 1911.