The word "蹑手蹑脚" is formed by repeating the verb "蹑" (meaning to tread or step lightly) in conjunction with "手" (hand) and "脚" (foot), creating a parallel compound where each instance of "蹑" modifies a separate body part. This structure directly illustrates the action of moving both hands and feet softly, resulting in a literal sense of walking stealthily or with caution.