The word desolate, with the same definition of "overcome by grief", was borrowed along a slew of other Latin terms when the language really began to reach England in the fourteenth century. The term in question, desolatus, is a past participle of the verb desolare, meaning "to leave alone" and coming from the prefix de- ("completely" or "thoroughly") and the root solare ("to make or become lonely"). De- meant "from" in an earlier time, and rather simply comes from a Proto-Indo-European reconstruction with basically the same spelling and denotation. Solare hails from the noun solus, "alone", which comes from a reflexive Proto-Indo-European root referring to the self, swe. The development of a verb form of desolate meaning "to abandon or ruin" came several decades after the noun was first attested in English. Usages of both desolate and desolation have closely paralleled each other as they decreased from relative peaks in the middle of the 1800s. Today, more people search for desolation on Google, but it's used less in literature.
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