An officious intermeddler is a person who voluntarily, and without request or pre-existing legal duty, interjects herself into the affairs of another, and then seeks remuneration for services or reimbursement. Example: Person "A" leaves for vacation for two weeks during the summer. Person "B" mows "A"s lawn. "B" requests payment for this service. Under common law doctrine "B" is not entitled to any payment from "A" beyond whatever "A" cares to give.
An exception to this rule is if a doctor gives medical treatment to an unconscious victim. The transaction costs for the doctor are high so the courts usually will rule this as restitution rather than officious intermeddler.
Another exception to the rule is when the recipient of the uninvited benefit subjectively would expect to pay the intermeddler for the services. For example, "A" has a driveway covered in snow after a recent blizzard. "B" shovels the snow from A's driveway. A observes B's activities and A thinks 'yes! free snow shoveling, I am not going to pay for B's valuable services.' A will owe B for B's services.
Second Restatement of Contracts Sec. 74
Restatement of Restitution Sec. 2
Restatement of Restitution Sec. 116
Black's Law Dictionary