Green’s Dictionary of Slang

1. a girl- or boyfriend.

1938		D. Runyon ‘Situation Wanted’ in Runyon on Broadway (1954) 660: Miss Anna Lark is my sweet-pea, and she loves me as dearly as I love her.
1944		D. Runyon Runyon à la Carte 3: Miss Dawn Astra is the sweet pea of a character by the name of Julius Smung.

2. a term of affection; also in ironic use.

1944		D. Burley Orig. Hbk of Harlem Jive 102: I dug on that trill play you was putting down when you were brooming into my life, sweetpea.
1949		I.L. Idriess One Wet Season 100: Don’t you disgrace me, you bunch of sweet-peas!
1976		P. Conroy Great Santini (1977) 470: I want the real reason, sweet pea.
1986		L. Heinemann Paco’s Story (1987) 13: Come awn, Sweet Pea, twiddle those goddam things in my moustache.
1996		A. Close Official and Doubtful 355: Sweetpea, I’m delighted.