What does armel stand for?

el stands for little-endian; see the email explaining (briefly) the decision; the follow-up emails contain more information. Endianness isn't the most distinguishing feature of armel, but that's the name that was chosen...

Further evidence is in Wookey's Debconf7 talk introducing the armel architecture; in the video at 26:47 he explicitly says "armel, basically little-endian ARM".

Other architectures using el in the same way include mipsel and ppc64el.

As pointed out by Kurt, el is the initials of "little-endian" read in little-endian order.

The e stands for EABI; the l for little-endian.