Thing To Be Today If Possible: septicidal.

 You may think that being septicidal involves killing things in groups of seven (the Latin septem, as I'm sure you know, meaning seven, and the Latin suffix -cidal usually being to do with killing things, the Latin -cīda meaning cutter or killer). 

But if septicidal did mean that, then of course The Word Den, an entity of impeccable probity, would not be recommending it.

On the other hand you may be fairly confident that being septicidal is really to do with preventing infection, as in the Greek word sēptos, which means decayed and, when spotted in an English word (antiseptic, for instance) is often to do with germs.

But in both cases you'd be misled, because septicidal is actually a botanical term and it means splitting along the partitions of a seed capsule.

A little learning is a dangerous thing.

But it's the most any of us ever have.

Thing Probably Impossible To Be Today: septicidal. The septi- bit of the word in this case is to do with the Latin word septum, which is a medical term for a division between two cavities or two pieces of tissue. Saeptum in Latin means wall. 

The -cidal bit means cutter, as above.