'Scheiss' as a somewhat shorter form of 'scheisse' (shit, crap)
Scheiss in general is often used the same way, as 'fuck' in US Englisch...
In German language, e.g. 'scheiss' (from Scheisse - shit) and 'egal' (equal) becomes 'scheissegal' which means whatever but with an 'fucking whatever' in mind.
So ein Scheiss - What a shit
Was für ein Scheiss hast Du gemacht? - What crap did you do?