only, free, in addition
只 centers on singling something out as the sole item or exception: only, just, or merely. The 'free' sense extends this to something given without anything else required, and 'in addition' can be seen as a contrasting use where something extra is added to the sole thing.
只 is thought to originate from a pictograph of a mouth with breath or sound coming out, later used abstractly for 'only' or 'just'. The exact historical development is uncertain.
The top 口 looks like a mouth, and the bottom strokes look like legs. Imagine a person standing alone, mouth open, saying 'only me!' — that's 只, meaning only or just.
For シ, imagine a single sheet of paper with 'only' written on it: sheet -> シ, and that sheet is the only thing there.
ordinary; common; usual
I'm home!; I'm back!; it's me!
middle
ordinary person
unusual; serious
just now
shikantaza; meditation practice where one stays intensely focused without focusing on any particular object
right in the midst of; right at the height of