godan verb (-ru)
keep watch; keep an eye on
Rare or dialectal verb meaning to watch over or keep a lookout; not commonly used in standard modern Japanese.
古老が畑をまばっていた。
The old farmer was keeping watch over the field.
見張る is the standard modern verb for 'keep watch' or 'stand guard'; まばる is a rare or dialectal equivalent.
The origin is uncertain; possibly a dialectal or archaic verb. The kanji 瞻 means 'look up' or 'gaze', but the word is usually written in kana.