'dotty' originates from English, formed from the noun 'dot' plus the adjectival suffix '-y', where 'dot' meant 'a small spot or mark'.

'dot' came from Middle English 'dotte' (also 'dot'), ultimately from Old English 'dott' meaning 'a small lump or spot'. The adjective 'dotty' developed in modern English by adding '-y' to mean 'full of dots' and later extended metaphorically.

Initially, it meant 'full of dots' or 'spotted', but over time it evolved metaphorically to mean 'slightly mad or eccentric' and also 'very fond of' in informal use.