1.
scum, foam, impurities
The foam or impurities that rise to the surface when boiling meat, vegetables, or other foods. Removing this scum (あく取り) is a basic cooking technique in Japanese cuisine.
2.
harsh or astringent taste, bitterness
The unpleasant bitter, astringent, or harsh taste found in certain vegetables and wild plants. Removing this harshness (あく抜き) is necessary before eating such ingredients.
3.
strong personality, pushiness
Used figuratively to describe a person with a strong, overwhelming, or abrasive personality that can be off-putting to others.
Writing
The kanji 灰汁 literally means "ash water" — historically, lye was made by filtering water through wood ash. This is an irregular (ateji) reading; the kanji cannot be read character by character to produce あく.
Usage
Often written in hiragana (あく) rather than kanji, especially in cooking contexts. The phrase 灰汁が強い is used across all three senses: strong scum, strong bitterness, or strong personality.