[from the dash that replaces the ‘am’ in dash, i.e. d—n, and subseq. in other ‘obscenities’; note Captain Alexander Smith, History of the Lives of the Most Noted Highwaymen (5th edn, 1719): ‘If we have here and there brought in some of these wicked offenders venting a profane oath or curse, which is dashed, it is to paint them in their proper colours’]
[late 18C+] a general euph. for damn v., used in excls. and oaths.
2. as past participle of sense 1; usu. as I’m dashed if... or I’ll be dashed...
dash it (all)!
[19C+] a general euph. excl.
dash my buttons! (alsodash my sconce! ...skin! ...timbers!, durn my buttons!)