party: [13] The Latin verb partīrī ‘divide up’ was derived from pars ‘part’ (source of English part). The feminine form of its past participle, partīta, was used in Vulgar Latin as a noun meaning ‘part, side’, and passed into English via Old French partie. This was later reinforced by Old French parti, which came from the Vulgar Latin neuter form *partītum and contributed the English word’s more salient current senses ‘political group’ and (in the 18th century) ‘social gathering’.
Other contributions made to English by the Latin past participle are the element -partite of words like bipartite, tripartite, etc and (via Italian) the musical term partita [19]. => part
