Title text: To anyone I've taken on a terrible date, this is retroactively my cover story.
Megan's sister has a crush on Cueball, but Cueball doesn't feel the same way about her sister. To alleviate the situation without hurting Megan's sister's feelings, Megan wants Cueball to deliberately take her sister out on a bad date to convince her that Cueball is not worth dating, so that her sister can move on without rejection. Cueball becomes extremely excited at the thought of deliberately having a bad date, especially because he has had some awkward dates in high school. The joke is that people often want their dates to go as well as possible, and this is a rare opportunity to do the exact opposite.
The title text implies that Randall is reflecting on previous bad dates. When they occurred he had either no excuse or a poor excuse for why he was so awkward, but now "deliberately being mediocre" is going to be used as his explanation. It is similar to thinking of a good comeback to an insult after the fact; this is his retort to ships that sailed long ago.
- [Megan on the phone.]
- Megan: I know you're not that into my sister, but she's really crushing on you.
- Cueball: Yeah, it's awkward.
- Megan: She's in a rough spot. It's a lot to ask, but could you take her out and... Dissuade her, without rejecting her?
- Cueball: Wait a second. Are you asking me to show her a mediocre time?
- Megan: I know it's a weird—
- [Cueball raising index finger.]
- Cueball: No, no! This is the mission I was born for.
- Megan: I figured you could handle it.
- Cueball: One of my classic high-school dates coming up!
- Megan: Oh God. Don't overdo it.