[CQ-Contest] origin of "es" in Morse code (cw)?

[Douglas Zwiebel]

Hi all,

I was just watching something on TV about the assassination of Lincoln and
noted that at one point in the show, the address (which I can't remember
exactly) was (say) "first and main," but it was listed at "1st es Main."

Boy did that get me thinking.

I tried Google, but nothing.

Any help?

de Doug KR2Q