*(s)lē̌b-, *(s)lā̌b-

The core idea of this root was:

weak, lack of tension, hanging down, gliding down

According to DWDS.de, the main etymological source I am using, it’s the root of sleep and the Latin labi, which was about weak, collapsing.
(English sources do not agree  with this connection and list the root of sleep as sleb-, but the core idea of weak is the same)

The Latin labi is the origin of words like lapse, collapse, labor and elaborate.
Other members are sleep and lip.