Literal translation
I would bet my head to be cut off
Meaning
I'd bet my life on it
Explanation
The expression serves to assert absolute certainty by symbolically staking what one holds most precious: one's own life. Literally, jougariéu ma tèsto à coupa translates as I would bet my head to be cut off: the verb jouga here carries the sense of to bet, to stake, while tèsto indeed names the head, not the verb to test as an erroneous homographic reading might suggest. The image recalls the old punishment of beheading, symbolically pledged to guarantee the truth of a statement: the speaker accepts, should they be wrong, the most extreme penalty imaginable. The register is elevated in intent, though used familiarly in everyday conversation to back an opinion forcefully, much like the French j'en mettrais ma tête à couper or I'd swear on my life. It marks total personal commitment behind the claim being made.