It seems Meretz is a second-order acronym – assembled from pieces of other acronyms, names of previous parties that Meretz was formed from.
The initial letter מ mem is from the initial of the Mapam party – the name מפ"ם Mapam is an acronym of מפלגת הפועלים המאוחדת Mifleget ha-Po‘alim ha-Me’uḥedet ‘United Workers Party’.
The next two letters ר resh and צ tsadi are the name of the Ratz party – itself an acronym of those two letters-- though why those two letters I don’t know, because the official name of the party itself (ha-Tenoa‘ah Lizkhuyot ha-‘Ezraḥ ve-Shalom) doesn’t use those letters. It was nicknamed Ratz because it was represented by the letters resh-tsadi on election ballots. No idea why that was. It forms the number 290.
מרצ Meretz itself is a word meaning ‘vitality’. Mapam is not a word in Hebrew though, just an initialism. רץ Ratz is a word meaning ‘runner, courier, bishop in chess’. But the party’s name is spelled רצ suggesting just the two letters as letters, not the word ratz, because it ends with the non-final form of tsadi, i.e. the tsadi is there as a letter on its own, not the final letter in an actual word.
Just to add to this very good analysis – Besides being the party’s “call letters,” as it were, at one point *Ratz *ran (heh) on the slogan “או שאתה **רצ **או שאתה הולך לאיבוד” (O she’ata ratz, o she’ata holekh l’ibud – “You either run, or you get lost [lose your way]”) – so *Ratz *רצ was definitely a meaningful acronym.
We do this occasionally in Quebec, not as assiduously as English-speakers, but a bit. I’m part of a housing rights group called POPIR - what the acronym stands for is long forgotten and is not actually relevant to its current mission (it’s been around forever in various incarnations), but it sounds like “pas pire” (“not bad”).