Scottish and, occasionally, Irish, derived, as is Hugh, from the Anglicised version of Gaelic Eóghan. Whyte gives the meaning as "youth" but, according to one correspondent [JE], it may be derived from "yew" [the yew tree, Taxus baccata]. See also Euan.
Ewan is occasionally found in the 1841 Census of Aberdeenshire where Ewen is the preferred, Scottish, spelling.
In old documents in Latin, Eugenius is used for Euan/Ewan/Ewen as well as for Eugene, with appropriate case endings.