'axillar' originates from Late Latin, specifically the word 'axillaris', where 'axilla' meant 'armpit'.

'axillaris' (Late Latin) derived from Latin 'axilla' ('armpit') and passed through Medieval Latin and Middle English forms to yield the modern English 'axillar'/'axillary'.

Initially it meant 'pertaining to the armpit'; over time this meaning has largely remained and was extended in botany to denote structures at a leaf axil.