Green’s Dictionary of Slang

weed n.3

1. an ill-conditioned, weak horse.

Surtees Hawley Grange (1926) 4: Two weed-riding, be-trousered, be-whiskered young gentlemen.
‘The Druid’ Post and Paddock 236: [He] expressed his astonishment that so old a sportsman should recommend him ‘a mere weed.’ However [...] the little fifteen-two ‘weed’ took six gates in succession.
[Aus]‘Rolf Boldrewood’ Robbery Under Arms (1922) 67: The only living things he cared about were Starlight and the three-cornered weed he rode.
[UK]‘Pot’ & ‘Swears’ Scarlet City 135: Wine women and weeds have been my ruin [...] I refer to racehorses.
[Aus]Sun. Times (Perth) 10 Jan. 2/3: Well bred though most of them are, they show little or no quality, and can easily be summed up as mean, undersized ‘weeds’.

2. a weakling, a feeble and thus contemptible person.

[US]Boston Globe Sun. Mag. 21 Dec. 7–8: The ‘sad bird’ or the ‘sad weed,’ the man who is not popular.
[Aus]Sun. Times (Perth) 16 Dec. 4/8: The ways of the weeds who take no tid.
[Aus]Bulletin (Sydney) 15 Dec. 30/2: Physically, he was a ‘weed,’ and his clothes seemed always to be too large for him.
[UK]T. Burke Limehouse Nights 294: Punditt regarded the weed in front of him with an airy tolerance.
[NZ]‘Anzac’ On the Anzac Trail 14: [A] weed many a time carries a bigger heart than a score of six-footers.
[UK]Marvel 3 July 13: I don’t think much of your new discovery [...] He seems to me a bit of a weed.
[UK]R. Westerby Wide Boys Never Work (1938) 132: A pink-skinned elegant little weed with curly hair.
[UK]G. Kersh They Die with Their Boots Clean 80: The muscle-factory, you weeds. The muscle-factory, you spineless gobs of calves’-feet jelly, you.
[UK]A. Buckeridge Thanks to Jennings (1988) 64: You are a couple of weeds pushing off like that.
[UK]E. Bond Saved Scene viii: I ain’ lettin’ a bloody little weed like you push me around!
[UK]P. Theroux London Embassy 71: That weed Beavis went to a birthday party there.
[Aus]T. Winton Human Torpedo 96: Lockie stood there. ‘Fight, weed.’.
[UK]Guardian G2 1 Mar. 14: The neurotic weed who once said he would never show his chest in public.

3. (Aus.) as sense 2, used derog. of a sports fan (seen as invariably physically inferior).

[Aus]Sydney Sportsman (Surry Hills, NSW) 26 Oct. 1/3: [T]he oft-complained-of habit which the ‘weed’ has of howling to his man to ‘'go in and finish the swine’ [...] The crowd of barrackers who haven’t a pound of muscle about [them].

4. (US Und./black/teen, also weed in the garden) a stranger, an outsider.

[US](con. 1905–25) E.H. Sutherland Professional Thief (1956) 243: Weed in the Garden – A stranger or any person regarded with suspicion.
[US]P. Crump Burn, Killer, Burn! 276: ‘What’s dis weed business?’ he asked. ‘A weed is a stranger, an outsider, It’s slang.’.