Not busy; lacking business or (useful) occupation.
Theobald's reading of The Tempest has been challenged by most editors; Stanley Wells and Gary Taylor's 1986 edition of the Compl. Wks. gives ‘busielest’; for other readings see their William Shakespeare: a Textual Companion (1987) 615/1..
Origin
Mid 17th century; earliest use found in Duchess of Newcastle (?1623–1673), writer. Probably from busy + -less.