say it: AHRM-pit
armpit is a noun meaning "the hollow under the arm where it is joined to the shoulder". It carries one recorded sense.
the hollow under the arm where it is joined to the shoulder
“they were up to their armpits in water”
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for armpit come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.