Green’s Dictionary of Slang

shepherd’s (plaid) adj.

bad.

1915		Dly Mirror 1 Sept. 10/3: ‘Not so shepherd’s’ is a shrtening of ‘not so bad’.
1932		‘P.P.’ Rhy. Sl. 11: Have a ‘butcher’s’ at her ‘ham and eggs’ – not so ‘shepherds’!
1960		J. Franklyn Dict. of Rhy. Sl.