'athermal' originates from the negative prefix 'a-' (from Greek 'a-/an-' meaning 'not') combined with 'thermal', which comes from Greek 'thermos' meaning 'heat' (via Latin/French forms into English).

'thermos' in Greek gave rise to Latin/Old French forms related to heat (e.g. 'thermicus'), which entered English as 'thermal'; the modern compound 'athermal' was formed in scientific English by adding the privative 'a-' to 'thermal'.

Initially constructed to mean 'without heat' or 'not heat-related'; over time it has come to be used both for 'not producing/involving heat' and for 'insensitive to temperature changes' in technical contexts.