Other forms: sweethearts
A sweetheart is somebody you love, or a term of affection for them. My sweetheart, childhood sweethearts, thanks, sweetheart.
Childhood sweethearts is the fixed phrase for a couple who met young. As a form of address it is warm from a partner, warm-but-familiar from a stranger, and can be patronising depending entirely on who says it to whom. Note sweetheart deal, meaning an arrangement unfairly favourable to one side, which is business language and quite unaffectionate.
1nounsweetheartSomebody you love, or a term of affection.
“Come here, sweetheart, and give me a hug.”
sweetheart is a term of affection.
Middle English swete hert is sweet plus heart, written as two words for centuries. The heart as the seat of love is an image English shares with most of Europe.
Middle English
swete heorte,swete hert
English, today
sweetheart
From Middle English swete heorte,swete hert; equivalent to sweet + heart.
Etymology from Wiktionary, CC BY-SA