From Latin 'imāgō' (image), related to 'imitārī' (to imitate) — imagination conceived as mental imitation of reality.
To form a mental image or concept of something not present or not yet existing; to suppose or assume.
From Middle English 'imaginen,' from Old French 'imaginer,' from Latin 'imāgināri' (to form a mental picture, to picture to oneself), from 'imāgō' (image, likeness, copy), genitive 'imāginis.' Latin 'imāgō' is thought to be related to 'imitārī' (to imitate, to copy), both possibly from PIE *h₂eym- (to copy). The connection between 'image' and 'imitate' suggests that to imagine was originally to create a mental copy of reality. Related Latin forms include 'imago' (image) and 'imitari' (to imitate), all from the same PIE root. Key
In Roman culture, an 'imāgō' was specifically a wax portrait mask of a deceased ancestor, kept in the family atrium and paraded at funerals. These masks were literal images — physical copies of a person's face. The leap from 'wax death mask' to 'mental picture' to 'creative invention' shows how a concrete, funerary object became the root of one of English's most abstract and creative verbs.