'astragal' originates from Late Latin, specifically the word 'astragalus', which in turn comes from Greek 'astragalos' meaning 'ankle bone' (also 'knucklebone').

'astragalos' in Greek passed into Latin as 'astragalus' and then into Middle English (via Medieval/late Latin usage) as forms like 'astragal(e)', eventually becoming the modern English word 'astragal'.

Initially it meant 'ankle bone' (and by extension 'knucklebone' used as a die); over time the sense was extended metaphorically to denote a small rounded molding or bead in architecture and carpentry.