Lank is South African slang meaning "very", "so", "extremely", "really". The slang lank is used to give emphasis, like an American would say "That is so lame!" Lank started among surfers but now everyone uses it.
Note: It is not "when you can lank" but "what sentences could you use the word lank appropriately".
Examples:
That guy is lank rude!
My soft drink is so lank warm it musta been stored near lava.
My parents are so lank strict I think they wear corsets.