I saw the abbreviation תושׁלכ"ע in the introduction to a textbook (Classical Mechanics, by Herbert Goldstein). What does it stand for?
Perhaps you saw תושלב״ע, rather than תושלכ״ע. As per this handy book of abbreviations, תושלב״ע stands for תם ונשלם, שבח לא-ל בורא עולם, “Whole and complete, praise to G-d, the Creator of the world.”
(The linked book does not include this abbreviation with a כ.)