Green’s Dictionary of Slang

arms and legs n.

also legs

weak beer or tea, i.e. a drink that has ‘no body’.

[UK]Farmer & Henley Sl. and Its Analogues.
[UK]Belfast News-Letter 11 Apr. 6/5: Weak beer with no body in it is dubbed ‘arms and legs’.
[UK]Yorks. Eve. Post 11 Nov. 20/4: Fivepenny mild is what is called ‘arms and legs’ — It’s without body.
[UK]‘William Juniper’ True Drunkard’s Delight 229: Of course, there is small beer or [...] arms and legs.
[UK]‘P.B. Yuill’ Hazell Plays Solomon (1976) 45: Fred! Make ’at anuvver free an’ two pintsa legs, myte.