IPA: /ˈslæʃɪŋli/
KK: /ˈslæʃɪŋli/
In a manner that involves cutting or slicing sharply.
The sword was swung slashingly, cutting through the air with great force.
'slash' (from Middle English *slaschen*, meaning to cut or strike) and '-ingly' (a suffix used to form adverbs indicating manner). The word 'slashingly' describes the manner of cutting or striking in a forceful or vigorous way.
Think of the action of 'cutting' ('slash') and how it is done 'in a manner' ('-ingly') — that's why 'slashingly' means to do something in a cutting or striking way.