The word "千万" is formed through the semantic combination of its two constituent characters, both of which are numerals representing vast quantities: "千" (qiān) meaning "thousand" and "万" (wàn) meaning "ten thousand". When paired, they do not simply add together arithmetically but instead merge conceptually to express an amplified sense of an extremely large, indefinite number, a "multitude" or "myriad". This fusion creates a single, cohesive idea of immensity or comprehensiveness, metaphorically representing "all the thousands and tens of thousands", which then extends abstractly to mean "be sure to" as an earnest exhortation based on that sense of exhaustive totality.