לְאַלְתָּר, לְאַלְתֵּר, אִלְתֵּר

There is a word לְאַלְתָּר in modern Hebrew, meaning "immediately". There is also another word that means the same: מיד, and all its relatives: מיידית, תכף ומיד. Anyway, the word לאלתר and the verb אִלְתֵּר/לְאַלְתֵּר (to improvise) sound to close to English word "to alter" (which originates from Latin), so it sounded suspicious to me, and I made some little research on the topic. It sounded like the Hebrew word comes from the meaning "to adjust, to change", with a slight semantic shift towards "to make immediate change."

Apparently, לאלתר has nothing to do with "altering". The word "alter" comes from the same Latin root as "alternative", and its general semantics is rather "another of the same kind", "different". The word לאלתר though comes from the Aramaic word אתר, with a preposition על: "(right) on (the) place": על אתר. This evolved (into עלתר and) into אלתר - and all this most probably happened in conversational language duric Mishnaic period, because in the new age no Hebrew scholars would allow guttural letters to drop out that easy.