Old English fot, from Proto-Germanic *fot (cf. Old Saxon fot, Old Norse fotr, Dutch voet, Old High German fuoz, German Fuß, Gothic fotus "foot"), from PIE *ped- (cf. Avestan pad-; Sanskrit pad-, accusative padam "foot;" Greek pos, Attic pous, genitive podos; Latin pes, genitive pedis "foot;" Lithuanian padas "sole," peda "footstep"). Plural form feet is an instance of i-mutation. Of a bed, grave, etc., first recorded c.1300.
The linear measurement of 12 inches was in Old English, from the length of a man's foot. Colloquial exclamation my foot! expressing "contemptuous contradiction" [OED] is first attested 1923, probably a euphemism for my ass, in the same sense, which dates back to 1796. The metrical foot (Old English, translating Latin pes, Greek pous in the same sense) is commonly taken as a reference to keeping time by tapping the foot.
To get off on the right foot is from 1905; to put one's best foot foremost first recorded 1849 (Shakespeare has the better foot before, 1596). To put one's foot in (one's) mouth "say something stupid" is attested by 1942; the expression put (one's) foot in something "make a mess of it" is from 1823.
c.1400, "dance, move on foot," from foot (n.). To foot a bill is attested from 1848, from the process of tallying the expenses and writing the figure at the bottom ("foot") of the bill.
n. pl. feet (fēt)
The lower extremity of the vertebrate leg that is in direct contact with the ground in standing or walking.
A unit of length in the U.S. Customary and British Imperial systems equal to 12 inches (30.48 centimeters).
Plural feet (fēt)
A unit of length in the US Customary System equal to 1/3 of a yard or 12 inches (30.48 centimeters). See Table at measurement.