Can terbinafine side effect dark line under toe?

No, but: The fungus in and under your nail that you're taking the terbinafine for certainly can produce this. Some fungi produce dark pigments. Show this to the doctor who prescribed the lamisil.

Under toe or nailbed: You probably mean dark line under the 'nail bed' (not the toe). If you do mean under the toe, is it the bottom of the toe? If so, terbinafine (lamisil) is not known to cause such side effect. As for dark line under the nail bed, this is something potentially serious. You need to see a foot specialist for a biopsy (in some cases, a dark line under the nail bed can turn out to be melanoma).