attach, connect, unite, fasten
繫 centers on making a physical or abstract link: attaching, connecting, uniting, or fastening things together so they stay joined.
繫 combines 糸 (thread) with a phonetic component that historically included 車 and 殳. The thread suggests tying or connecting, while the phonetic part likely contributed the sound.
The bottom 糸 is thread, and the upper part looks like a complex knot or a cart (車) with a tool (殳). Imagine using thread to tie a cart's parts together, fastening everything securely.
For ケイ, picture a heavy chain (K) being used to attach a cart (車) to a post: the 'K' sound cues ケイ, and the chain connects everything firmly.