A person in the office environment, who claims to put in 110% for the good of the office, can talk the talk, but struggles to walk the walk (or simply unable to achieve anything).
Peer response manifests as frustration or mocking of the said individual.
The Office Lightweight is usually a person from middle management.
Person A: "So I asked Kelvin for a day off, as my wife's in hospital. Bloody prick said no worries... my roster came out and had we working 25hrs that day....FFS"
Person B: "Yeah Kelvin is a real Office Lightweight. Next time just call in sick"