'deepen' originates from Old English, specifically the verb 'dēopan' (from adjective 'dēop' meaning 'deep') with the verbalizing suffix '-en'.

'dēopan' in Old English changed into Middle English forms such as 'depen'/'deepen' and eventually became the modern English verb 'deepen'.

Initially, it meant 'to make or become deep' in a physical sense; over time its use expanded to include figurative senses such as 'to intensify' or 'to make more profound'.