IPA: /ˈɡlæs.fəl/
KK: /ɡlæsˌfʊl/
The amount of liquid that a glass can contain when it is full.
She poured a glassful of water for her friend.
Glassful → It is formed from "glass" (from Old English *glæs*, meaning a transparent material) and "-ful" (meaning full of). The word "glassful" means full of glass or the amount that a glass can hold.
Think of a container that is 'full of' ('-ful') a 'transparent material' ('glass') — that's what a glassful means.