follow-through (n.)

Middle English folwen, from Old English folgian, fylgian, fylgan "to accompany (especially as a disciple), move in the same direction as; follow after, pursue, move behind in the same direction," also "obey (a rule or law), conform to, act in accordance with; apply oneself to (a practice, trade, or calling)," from Proto-Germanic *fulgojanan (source also of Old Saxon folgon, Old Frisian folgia, Middle Dutch volghen, Dutch volgen, Old High German folgen, German folgen, Old Norse fylgja "to follow"). Probably originally a compound, *full-gan, with a sense of "full-going," the sense then shifting to "serve, go with as an attendant" (compare fulfill). Related: Followed; following.

Sense of "accept as leader or guide, obey or be subservient to" was in late Old English. Meaning "come after in time" is from c. 1200; meaning "to result from" (as effect from cause) is from c. 1200. Meaning "to keep up with mentally, comprehend" is from 1690s. Intransitive sense "come or go behind" is from mid-13c. To follow one's nose "go straight on" first attested 1590s. "The full phrase is, 'Follow your nose, and you are sure to go straight.' " [Farmer]. The children's game follow my leader is attested by that name from 1812 (as follow the leader by 1896).

late 14c., metathesis of Old English þurh, from Proto-Germanic *thurx (source also of Old Saxon thuru, Old Frisian thruch, Middle Dutch dore, Dutch door, Old High German thuruh, German durch, Gothic þairh "through"), from PIE root *tere- (2) "to cross over, pass through, overcome." Not clearly differentiated from thorough until early Modern English. Spelling thro was common 15c.-18c. Reformed spelling thru (1839) is mainly American English.