An archaic or rare adjective meaning full of or causing ghastliness, horror, or terror.
From ghast (to terrify) plus the suffix -ful meaning 'full of.' This is an older formation that has largely been replaced by ghastly in modern English, though the construction follows standard English word-building patterns.
Ghastful is a 'dead word'—it existed but was outcompeted by ghastly, which became the standard adjective form. Language is like evolution: words with better 'fitness' survive while others go extinct.
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