mentalese
A hypothetical non-verbal language in which concepts are represented in the mind.
uncountable
The concept of a proposition as something that can be expressed by sentences in both Mentalese and, say, English is an analogical extension of the concept of a proposition as something that can be expressed by sentences in both English and German.
Where it comes from
From mental + -ese (suffix forming adjectives and nouns describing, among other things, languages).
Adapted from the Wiktionary entry mentalese (CC BY-SA 4.0).