Green’s Dictionary of Slang

1. untrustworthy, weak .

1815

(con. 18C) W. Scott Guy Mannering (1999) 178: I doubt Glossin will prove but shand after a’, mistress, [...] but this is a gude half-crown ony way.

2. unsteady.

1993

I. Welsh Trainspotting 6: Ah soon started tae feel fucking shan n aw.

3. general neg. used of individuals, objects and situations.

2012		(con. 1980s) I. Welsh Skagboys 435: A fried egg [...] would have utterly shan wi mashed tatties n neeps.
2012		J. Fagan Panopticon (2013) 156: Malcolm’s a shan name.
2014		Time Out 28 Oct. 🌐 11 essential Edinburgh slang terms [...] 1. 'Shan' Definition: Unfair, disappointing, terrible, bad. Often used in conjuction with ‘pure’ (ie ‘very’) to denote extreme shanness.
2016		V. McDermid Out of Bounds (2017) 106: If you saw inside their bedrooms you’d think we’d had a visit from extreme burglars. Totally shan.