'annular' originates from Latin, specifically the word 'annulus', where 'annulus' meant 'little ring'.

'annular' developed from Latin 'annulus' to Late Latin 'annularis' and entered English via Medieval/Modern Latin as 'annular'.

Initially it referred to a 'little ring' ('annulus'); over time the sense broadened to describe something 'ring-shaped' (adjective 'annular') and then 'in a ring-shaped manner' (adverb 'annularly').