In the RASC in my National Service time, 1951-52, it was always "Fuck my second-best boots", & was a sort of phatic expression of surprise or satisfaction.

A use of the phrase in a folksong, often a little euphemised, is in Ratcliffe Highway: e.g. version in Penguin Book of English Folk Songs (Vaughan Williams & Lloyd): "A gold watch hung over the mantel, So the change for my guinea I take, And down the stairs I run nimbly, Saying 'Darn my old boots, I'm well paid" ~~ tho Frank Purslow I recall singing it a little differently, & rather better rhymed ~ "The gold watch hanging over the mantel as change for my guinea I seized, Then nimbly I nipped down the stairs, crying dang me old boots, I'm well pleased".

~Michael~