'self-indulgently' originates from English, formed by the adjective 'self-indulgent' + the adverbial suffix '-ly'. 'self' comes from Old English 'self' meaning 'one's own', and 'indulgent' comes ultimately from Latin 'indulgent-' (from 'indulgere') meaning 'to be lenient or give way'.

'indulge' comes from Latin 'indulgere' (to be lenient, give way) and entered English via Old French and Middle English; 'indulgent' was formed as an adjective in English, then compounded with 'self' to make 'self-indulgent', and finally '-ly' was added to form the adverb 'self-indulgently'.

Initially related to the idea of 'being lenient or giving way' (in general), but over time 'self-indulgent' came to emphasize giving oneself excessive pleasure or gratification; 'self-indulgently' thus developed the sense of 'in an overly indulgent way toward oneself'.