الشَّفْعُ ضِدُّ الْفَرْدِ
Shafʿ, a pair, is the opposite of a single or unpaired thing.
The lexical idea is joining another in support. A person has a need or plea; someone else adds a supporting request on that person's behalf. Technically this may involve requesting pardon, relief from punishment, or a benefit.
The lexical derivation explains the form of intercession, not a power of compulsion over Allah. Acceptance remains subject to divine permission and will.