aplomb

the word 'aplomb' entered english in the late 18th century from the french phrase 'à plomb', meaning 'according to the plumb line' or 'perpendicularly'. this french term derives from the latin 'plumbum', meaning 'lead', the metal from which a plumb bob, a tool for establishing a true vertical line, was traditionally made. the metaphorical leap from physical uprightness and stability to mental and emotional poise is a classic example of semantic evolution, where a concrete concept gives rise to an abstract quality. thus, the modern sense of cool self-assurance evolved from the idea of being perfectly balanced, as straight and steady as a plumb line.

this word is a loanword from french, adopted as a simplex unit into english. it does not contain separable morphemes within its modern english spelling.