Plural of 'heinie,' a slang term for buttocks; also historically used as a derogatory term for German soldiers.
The buttocks sense likely derives from a variant of 'hiney' or 'heiney.' The military slang for Germans comes from 'Heinie,' a diminutive of Heinrich (Henry), a common German name used as a stereotyping nickname.
This word shows how soldiers create insulting nicknames by taking regular people's names and shrinking them—it's the same linguistic impulse that created 'Tommy' for British soldiers and 'Tojo' for Japanese military leaders.
Complete word intelligence in one call. Free tier — 50 lookups/day.