robin's unpleasant associations?

From The Cuckoo's Calling:

"Sorry I kept calling you Sandra; she was the last girl. What's your real name?"
"Robin."
"Robin," he repeated. "That'll be easy to remember." He had some notion of making a jocular allusion to Batman and his dependable sidekick, but the feeble jest died on his lips as her face turned brilliantly pink. Too late, he realize that the most unfortunate construction could be put on his innocent words. Robin swung the swivel chair back towards the computer monitor, so that all Strike could see was the edge of a flaming cheek. In one frozen moment of mutual mortification, the room seemed to have shrunk to the size of a telephone kiosk.

What on earth is that most unfortunate construction?

ETA: If you're English, "Robin" suggests "Redbreast", which reminds these two that when they met, he accidentally grabbed her breast. To keep her from falling down the stairs.