The word 'thief' originates from the Old English 'þēof', which is related to the Old Norse 'þjófr' and the Gothic 'þiubs'. These all trace back to the Proto-Germanic 'þeubaz', meaning 'thief' or 'robber'. This Germanic root is further connected to the Proto-Indo-European base 'teup-', meaning 'to swell', which may have developed a metaphorical sense of 'to be puffed up with secrecy' or 'to creep stealthily'. The semantic journey from a concept of swelling or creeping to one who steals covertly illustrates the evolution of abstract meaning from a more physical root.

The word 'thief' is a simplex word of Germanic origin. Its core form is not productively divisible into smaller, meaningful morphemes in modern English.