孟 is a bound morpheme (b.f.) that never stands alone as an independent word. It carries three distinct meanings depending on the compound it appears in: (1) 'first month of a season', as in 孟春 (mèngchūn 'early spring'); (2) 'eldest among siblings', used in the classical ordering 孟仲叔季 (mèng zhòng shū jì); (3) a common Chinese surname, as in the poet 孟浩然. Because it is a bound form, always use it as part of a two-character word or proper name.