foot vs foot up

foot is a noun but foot up is not a noun.

Word	Noun	Adjective	Verb	Adverb
foot	Yes	No	Yes	No
foot up	No	No	Yes	No

As verbs, foot and foot up are synonyms defined as:

foot (noun)	foot up (noun)
travel by walking
any of various organs of locomotion or attachment in invertebrates
the pedal extremity of vertebrates other than human beings
lowest support of a structure
a support resembling a pedal extremity
the part of the leg of a human being below the ankle joint
(prosody) a group of 2 or 3 syllables forming the basic unit of poetic rhythm
an army unit consisting of soldiers who fight on foot
the lower part of anything
a member of a surveillance team who works on foot or rides as a passenger
a linear unit of length equal to 12 inches or a third of a yard