Why is the ninth month called September?
1796, from Latin intumescentem (nominative intumescens), present participle of intumescere "to swell up," from in- "in" (see in- (2)) + tumescere (see tumescence).
1650s, from French intumescence, from Latin intumescere (see intumescent).
intumescence in·tu·mes·cence (ĭn'tōō-měs'əns, -tyōō-)n.
The act or process of swelling or the condition of being swollen.
A swollen organ or body part.