"Sorry for lifting bro" (SFLB) is statement used to respond to
your friends that are trying get you to engage in other activities than
going to the gym to training.
"Sorry for lifting bro" can also be used as a rebuttal to jealous comment from other gym goers that make negative comments about you physique to make them selves feel better because they cannot put in the hard work required to achieve similar results.
In short, its a kind way to tell those that are trying to distract or discourage you from reaching your fitness goals to, "go f#$& themselves" and
no matter what they say or do, they will not be able to keep you from reaching your fitness goals.
Friends: "you should skip the gym and we should go out and get wasted"
You: "Sorry for lifting bro" Then
take a picture of yourself
working out while they are partying and send it to them with the caption "sorry for lifting bro"
Guy in the gym: "you're only
bigger than me because you have better genetics"
You: "Sorry for lifting bro"