Ghastly means horrifying, or very unwell in appearance. A ghastly accident, look ghastly, a ghastly mistake.
Of a scene it means genuinely horrifying, and of a person's appearance it means pale and ill — you look ghastly. In British informal use it is also a mild complaint about anything unpleasant: a ghastly meeting, ghastly weather. Note that the h in ghastly was inserted by analogy with ghost, which is a separate word, and that ghost itself got its h from a Flemish printer's habit.
1adjectivehorrifyingHorrifying.
“Police found the ghastly remains of the victims in the cellar.”
2adjectiveghastlyVery unwell in appearance.
“You look ghastly; you should go home and lie down.”
Too few real examples of this word to estimate which sense is commonest.
ghastly is a quality of horror or illness.
Old English gaestan meant to frighten. The h was added in the sixteenth century by association with ghost, which had itself acquired one from Caxton's Flemish typesetters — so two words share a silent letter for no reason at all.
Old English
gǣstan — to torment, frighten
Middle English
gastly
English, today
ghastly
From a conflation of gastly, from Middle English gastly, from gasten (from Old English gǣstan (“to torment, frighten”)) + -ly, and ghostly (which was also spelt gastlich in Middle English). Equivalent to ghast/gast + -ly. Spelling with gh developed in the 16th century due to the conflation.
Etymology from Wiktionary, CC BY-SA