'astragalar' originates from Greek, specifically the word 'astragalos', which passed into Latin as 'astragalus', where 'astragalos' meant 'ankle bone' (a small rounded bone).

'astragalos' (Greek) became Latin 'astragalus', entered Medieval and Modern Latin and then English as 'astragalus'; the adjective form 'astragalar' developed in English to mean 'relating to the astragalus' and by extension 'relating to an astragal (moulding)'.

Initially it referred directly to the 'ankle bone' itself; over time derivative forms came to mean 'relating to that bone' and were also applied by analogy to small rounded mouldings (astragals) in architecture and carpentry.