Foot has many meanings, the most common of which are (1) a part of the body and (2) a unit of length. The plural of foot is feet. The singular is preferred in such expressions as "a 3-foot ruler'' and "a 9-foot wall," despite the fact that the numerals involved indicate more than one. However, idiom requires that one refer to "a ruler 3 feet long" and "a wall 9 feet high." One can say "a 6-foot man" but should say "a man 6 feet tall." That is, foot is normally used in forming compound adjectives (barefoot girl), and footed (not feeted) is employed in such terms as "four-footed" and "sure-footed." Foot appears in such trite phrases as "put one's best foot forward," "put one's foot in one's mouth," "always underfoot," "footloose and fancy-free," "get off on the wrong foot," "have one foot in the grave," "put one's foot down," "put one's foot into it." Feet is tiresomely used in "set someone on his feet" and "feet first."
Dictionary of problem words and expressions. Harry Shaw. 1975.