The Spanish word 'alegre' meaning 'happy, cheerful' comes from the Vulgar Latin '*alicer', which meant 'lively, cheerful'. This in turn descended from the Classical Latin word 'alacer', meaning 'quick, lively, animated'. The semantic development from 'quick, lively' to 'cheerful, happy' is quite natural, as physical liveliness and emotional happiness often go hand in hand.

Some related Spanish words include 'alegría' (happiness, joy), 'alegremente' (cheerfully), and 'alegrar' (to make happy, to cheer up). All these words share the same Latin root and form a family of words expressing happiness and cheer in Spanish.

While English doesn't have any direct cognates from the Latin 'alacer', we do have the word 'alacrity', which was borrowed from Latin and means 'cheerful readiness, liveliness, or briskness'. This preserves more of the original Latin meaning of quickness or liveliness, while the Spanish 'alegre' has shifted more towards the emotional aspect of cheerfulness.