It was Robin's jaw-dropping beneath-the-belt bulge.
Burt Ward is, by all accounts (including his own), massive. He won't give his exact measurements, but I'm guessing Mortadella.
It was hard to cram him into that Robin Hood costume without his something extra showing.
Especially when he was tied up, struggling to escape from the latest diabolical trap.
Which happened in nearly every episode.
Check out these two pictures. As the ropes get tighter, Robin gets warmer.
Well, he couldn't help it. Burt Ward was in his early 20s, and he often had to spend an hour at a time restrained, with nothing to do but wait. Extras and guest stars often took advantage of the opportunity to play with him.
Female extras, he claims. I'm not so sure.

I also have a correspondent who claims to have hooked up with Burt right on the set.
About a dozen episodes into the first season, a "Save the Children" watchdog group complained, and the directors and crew found ways to underplay Burt's package. Or hide it altogether.
But it remains the stuff of legend.
See also: Lane's Hookup with Batman, Robin, and the Joker.; A Hookup with Robin the Boy Wonder