When we were kids, we would sit sniggering in the big hall during assembly because someone had 'let one off' It was otherwise known as a silent but deadly...an SBD
So, just like when you think of the US president and you snigger at his very apt name, you can snigger at the silent but deadly's that are lingering in your wallet...
Ewwwwwwww...
I am afraid to say I am at times still that sniggering girl in the assembly hall every time I see the initials SBD