Frightening or alarming; causing fear or terror (Scottish and Northern English dialect).
From fley (to frighten) + -some (adjective suffix meaning 'characterized by' or 'tending to'). The -some suffix creates adjectives describing qualities, as in 'tiresome' or 'loathsome.'
The suffix -some originally meant 'bodily' in Old English, but evolved to describe tendencies—so 'fleysome' is literally something that has the tendency or nature of causing fleying!
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