'smooth-tailed' originates from English, formed by combining the adjective 'smooth' (Old English 'smōþe', meaning 'smooth') with the noun 'tail' (Old English 'tægl', meaning 'tail') plus the adjectival '-ed' suffix to indicate 'having a tail that is smooth'.

'smooth' comes from Old English 'smōþe' and 'tail' from Old English 'tægl'; the compound adjective 'smooth-tailed' is a Modern English formation created by joining these elements with '-ed' to describe a physical characteristic.

Initially it described the simple physical property 'having a smooth tail'; over time it has remained a straightforward descriptive term, mainly used in zoological or descriptive contexts.