*klei-

The core idea of this root was:

leaning, to be at an incline

This sense is still pretty obvious in words like to lean, to incline or to recline, but also clinic or client come from that root.  The origin of these is an old Greek word with the simple sense of “laying in bed sick”.
Here’s an (incomplete) list of English relatives:

• to lean 
• to incline (make leaning)
• to recline (to lean back)
• to decline (to lean away from an offer)
• clinic (place where people “lean” in bed)
• client (person “leaning” in bed)
• climate (originally: region/slope of earth)
• climax (an sloping up of intensity)
• ladder (leaning against wall)