The adverb tenuously means in a weak or insubstantial way. If you tenuously understand French, you're probably not fluent enough to have a meaningful conversation with your Parisian cabdriver.
Tenuously and the adjective tenuous share the Latin root tenuis, or "thin." Things that are done tenuously have a literal or figurative thinness to them. If you're tenuously disguised as your dad, you may simply be wearing his favorite hat — the masquerade is truly insubstantial. And if a politician tenuously links an opponent's policies to an economic downturn, the connection is also too thin to be trusted.