'fluid-packed' is a compound formed from 'fluid' and the past-participle adjective 'packed' (from the verb 'pack'), where 'fluid' comes from Latin 'fluidus' meaning 'flowing' and 'packed' derives from the verb 'pack' meaning 'to place together tightly'.

'fluid' originates from Latin 'fluidus' (from the verb 'fluere' meaning 'to flow'), which passed into English via Middle French/late Latin; 'pack' came into English via Middle English (e.g. 'pakken') from Old Norse 'pakka' or related Germanic sources, and 'packed' is the past participle form used attributively in compounds like 'fluid-packed'.

Individually, 'fluid' originally meant 'flowing' and 'pack' meant 'to place together'; combined as 'fluid-packed' the meaning evolved to denote something that is filled or densely occupied by fluid ('filled with liquid').