The term 'COMPLAIN, SO TO SPEAK' can be solved by two crossword solutions: GROAN (5 letters) and MOAN (4 letters). Groaning and moaning are both forms of vocal expression that are used to express displeasure or dissatisfaction. They are often used to express frustration, pain, or sadness. Groaning is usually a louder and more drawn out sound than moaning, which is usually a softer and shorter sound. Both are considered to be non-verbal forms of complaining.