IPA: /ˈlæŋk.nəs/
KK: /læŋk.nəs/
The quality or state of being tall and thin, often in a way that is unattractive or awkward.
His lankness made him look taller than he actually was.
Lankness is formed from "lank" (from Old English "lanca", meaning thin or slender) and "-ness" (meaning the state or quality of). The word "lankness" refers to the state of being thin or slender.
Think of 'lank' meaning thin, and '-ness' indicating a state. So, lankness helps you remember the quality of being thin.