Archaic humble first-person pronoun used by men. In modern Japanese, it is only encountered in historical contexts, period dramas, or deliberately old-fashioned speech.
"I am but a mere traveler," said the samurai.
Kana spelling; may be encountered in historical texts or explanations.
The historical derivation is uncertain. The kanji 僕 originally meant 'servant' or 'manservant', and the reading やつがれ likely developed as a humble self-reference.