'poetical' originates from Latin and Greek, specifically ultimately from Greek 'poiētikos' (ποιητικός), where 'poiētēs' (ποιητής) meant 'maker' or 'poet' and 'poiein' (ποιεῖν) meant 'to make' or 'to compose'.

'poetical' changed from Greek 'poiētikos' to Latin 'poeticus', passed into Old French as 'poetique'/'poetical', then into Middle English and eventually became the modern English word 'poetical'.

Initially it meant 'of or relating to a maker/poet', and over time it evolved into the modern sense 'relating to poetry' and, by extension, 'having poetic or imaginative qualities'.