Rainbow Dash is unexpectedly naive.
 
For those of you who missed the discussion on the original Japanese posting, this translation has been massaged a bit to help the joke work in English. In the original dialogue, RD says いや～、昨日も頑張ったんだけど、中折れしちゃってさ～。 Now, this literally translates as “Aw, man, I tried my best yesterday, but I totally (nakaore)!”
 
Nakaore means “to fold halfway”, but has the slang meaning of “to go soft during sex”. It’s a bit funnier in the original, since the first panel seems to have her saying, “I tried my best, but my dick went totally limp, so I wasn’t able to finish.” She’s mixed it up with the word nakadarumi (中だるみ), which means “to slack off”. The problem here is that English doesn’t have a convenient word for going soft that sounds similar to slacking off. So, I bit the bullet and changed the dialogue slightly to something that’s still sexual in nature, but still works with the rest of the joke.