Green’s Dictionary of Slang

1. whisky [pee n.¹ (1) ? SE peat, as in modern brand-name Johnnie Walker Sweat Peat; note Partridge suggests ‘the colour of the resulting urine’, i.e. pun on pee n.¹ (1), but no reason for ‘sweet’; ].

1795		‘A Peep into Whiskey-Shop’ in A. Carpenter Verse in Eng. in 18C Ireland (1998) 518: We stept into Maddins for certain, sir fill a half pint of sweet pea.
1821		‘A Real Paddy’ Real Life in Ireland 17: Brian [...] drank sweet pea (whiskey) with Poll Codee the wig-seller.

2. urine [pee n.¹ (1)].

1890–1904

Farmer & Henley Sl. and Its Analogues.

3. (Aus.) a knee.

1914

N.Z. Truth 31 Jan. 2/8: I fell down the apples and pears [...] and busted the sweet peas on my terrace of houses (bursted the knees of my trousers).

4. tea.

1960, 1975		Wentworth & Flexner DAS 648/3: sweet pea – tea.
1981		J. Franklyn Dict. Rhy. Sl. (2nd edn).

5. LSD.

1978

L. Kramer Faggots 301: You name it, somebody’s on it. [...] Blotter, Orange Sunshine, Sweet Pea.