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 is excited to finally excel at dating. He claims to have been born for the task of giving a bad date. He promises to emulate one of his classic high school dates, indicating they went terribly as well. The joke is based on the reversal of expectations and Cueball's subsequent excitement at the chance to provide a terrible date. Megan's respose of "Oh God, don't overdo it" may also imply that she was one of Cueball's high school dates and doesn't want her sister to be subjected to what she remembers of Cueball's relationship skills
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.