IPA: //ˌʌndərˈteɪkɪŋ//
KK: /ˈʌndərˌteɪkɪŋ/
A task or project that someone decides to do or take on, often involving effort or responsibility.
Starting a new business is a big undertaking that requires careful planning.
Undertaking is formed from "under-" (meaning among or between) and "taking" (from the Old English "tacan", meaning to take). The word refers to the act of taking on a responsibility or task, often implying a commitment to complete it.
Think of 'taking on' a task or responsibility, where 'under-' suggests being among the tasks you are taking.