certainly, (kokuji)
聢 centers on certainty and firmness. It is a kokuji (Japan-made kanji) used in the adverb しかと (shikato) meaning 'certainly' or 'firmly'.
聢 is a kokuji created in Japan, combining 耳 (ear) and 定 (determine, fix) to express the idea of hearing something firmly or certainly. The exact historical development is uncertain.
An ear 耳 that fixes 定 on a sound hears it certainly. Picture an ear locking onto a sound with absolute certainty.
For しか, imagine a deer (鹿, しか) that hears a sound with its ear 耳 and fixes 定 on it certainly: しか -> しか.