The term 'sink in' can be described by four crossword solutions: REGISTER (8 letters), PENETRATE (9 letters), INFILTRATE (10 letters), and MIRE (4 letters). REGISTER means to record or enter information into a system, while PENETRATE means to enter or pass through something. INFILTRATE is similar to PENETRATE, but it implies entering something secretly or without permission. Lastly, MIRE means to become stuck in a difficult situation. All of these words describe the phrase 'sink in, ' which means to become understood or accepted gradually.