'shell-less' is a compound of 'shell' and the suffix '-less'. 'shell' originates from Old English, specifically the word 'scell' (or 'scel'), where the root meant 'a husk or outer covering'. The suffix '-less' originates from Old English 'lēas', meaning 'without'.

'shell' developed from Old English 'scell' through Middle English forms such as 'schel'/'shel(e)' into the modern English 'shell'. The suffix '-less' comes from Old English 'lēas' and evolved into the productive Modern English suffix '-less' used to form adjectives meaning 'without X'. The compound 'shell-less' therefore formed in Modern English by combining these elements.

Initially the components meant 'outer covering' and 'without'; combined they have meant 'without an outer covering' (i.e., 'without a shell') and this core meaning has remained stable in modern usage.