'flammably' originates from Latin, specifically the word 'flamma', where 'flamma' meant 'flame'.

'flammably' changed from the Latin adjective 'flammabilis' (meaning 'able to catch fire'), passed into French/Medieval Latin forms and the English adjective 'flammable', and eventually formed the adverb by adding the English adverbial suffix '-ly' to produce 'flammably'.

Initially it meant 'able to catch fire' (applied to substances); over time the adverbial form came to mean 'in a manner that can burn easily'.