Refers to a physical opening designed for inserting something, such as a mail slot, coin slot, or ballot box opening.
I put the letter through the mail slot.
この機械の差し入れ口に硬貨を入れてください。
Please insert coins into the slot of this machine.
Standard spelling for a slot or opening through which something is inserted.
Variant spelling; less common but still used.
Compound of 差し入れ (sashiire, 'inserting') + 口 (kuchi, 'mouth, opening'). The exact historical derivation is uncertain, but the word transparently describes an opening for insertion.