Learn English – Slang or idiom for someone who wants to gain weight or bulk up

My insipid smile is a very watery smile. You might say it's a washed out smile.

Websters/ARTFL Insipid:

Wanting in spirit, life, or animation; uninteresting; weak; vapid; flat; dull; heavy;

Collins: Watery:

4. insipid, thin, or weak

Merriam Webster Washed out:

2: depleted in vigor or animation

The classic idiom in English is "third wheel". This can be used both against both parties (the couple and the third party). For example:

The couple might say: "He is such a third wheel. I wish he'd stop trying to hang out with us."

The third party might say: "They make me feel like such a third wheel. I don't understand why they invite me along when they clearly don't want to hang out with me."

A more pejorative (and much more vulgar) term might be "cockblock". The implication here is that the intruder is preventing the couple from "getting it on" (having sex) by blocking the male's cock (penis).

"Tommy is such a cockblock. He needs to get a life."

However, cockblock isn't necessarily only about stopping sex. In fact, cockblock has been expanded to include any individual that stops something that people want to do, especially in online gaming:

"Tommy is such a cockblock. I almost got to that extra-ammo pickup, but he killed me before I could. He needs to get a life."