adverb
Literary adverb used before a number to give a rough estimate. Often found in formal or written contexts; in everyday speech 約 (やく) or 大体 (だいたい) are more common.
無慮三千人の観客が集まった。
Approximately three thousand spectators gathered.
その城は無慮五百年の歴史を持つ。
The castle has a history of some five hundred years.