"If" poem by Rudyard Kipling

make allowance for -- accept/tolerate/forgive
keep your head -- stay in control/don't go crazy
doubting -- unbelieving/not trusting
deal in -- involve in/take part in
wise -- clever or with wisdom
triumph -- winning, success or victory
disaster -- failure or tragedy
impostor(s) -- pretender/fake
bear -- stand/accept
knave(s) -- cheater/dishonest person
stoop -- to bend the body forward or backward
build 'em up = build them up
worn-out -- over-used/exhausted
heap -- a large collection of something/a pile
pitch-and-toss -- "throw--and--flip" of a coin/gamble
nerve -- (metaphorically) courage and control under pressure
sinew -- muscle, (metaphorically) strength/power
will -- determination/spirit
virtue -- good value/quality
common touch -- ability to communicate with ordinary people.
foes -- enemies