Naissant, like the more familiar rampant, is a term used to describe the pictures painted on shields and coats of arms.
Here's a shield with a lion naissant on it:
That's the Coat of Arms of the municipality of Ramillies. Illustration by Evil berry
Here's another example, a stag naissant, this time:
Illustration of the Coat of Arms of Macorquodaill of that Ilk by Czar Brodie.
As you can see, naissant means appearing to be cut-off at the waist.
These days, the thing that makes people appear to be cut-off at the waist is all too often a desk...
...which makes the word naissant's derivation just deeply, deeply ironic.
Thing Not To Be All Day Today: naissant. This word comes from the Old French, where it means being born.
Having said all this, it occurs to me that sitting at a table to eat a meal is one of life's greatest pleasures, and also makes you look naissant. So I wouldn't want to volunteer for an entirely naissant-free day.