here's an entry for diabolical from OED2 which you may find to be helpful:

3. slang. In weakened sense: outrageous, disgraceful; disgracefully bad. Also, used as an intensive, esp. in diabolical liberty.
1958 B. BEHAN Borstal Boy III. 231 Why ain't I given a chance to follow my trade in 'ere, eh?..It's a diabolical liberty. Geezers get no chance to follow their trade. 1965 Listener 30 Sept. 507/2 A parody piece..which took diabolical liberties with Eurovision song contests, British Beatlemania and other suitable themes. 1972 MITTON & MORRISON Community Project in Notting Dale 69 The names he called those kids round there was diabolical for any vicar. 1982 S. TOWNSEND Secret Diary A. Mole 94 Asked our postman about communications between Tunisia and England. He said they were ‘diabolical’. 1986 Observer 16 Feb. 50/4 From my point of view that pitch was dangerous. In fact, it was diabolical.