Wiktionary:Word of the day/2021/December 19

1. (also figuratively) Of fruit: soft or tender by reason of ripeness; having a tender pulp.
2. (also figuratively) Of food or drink, or its flavour: matured and smooth, and not acidic, harsh, or sharp.
3. Of soil: soft and easily penetrated or worked; not hard or rigid; loamy.
4. (chiefly poetic)
   1. Of leaves, seeds, etc., of plants: mature; of crops: ready to be harvested; ripe.
   2. Of a place, or the climate or weather: fruitful and warm.
5. (figuratively)
   1. Of colour, sound, style, etc.: not coarse, brash, harsh, or rough; delicate, rich, soft, subdued.
   2. Senses relating to a person or their qualities.
      1. Well-matured from age or experience; not impetuous or impulsive; calm, dignified, gentle.
      2. Cheerful, genial, jovial, merry; also, easygoing, laid-back, relaxed.
      3. Drunk, intoxicated; especially slightly or pleasantly so, or to an extent that makes one cheerful and friendly.
      4. (chiefly US, slang) Pleasantly high or stoned, and relaxed after taking drugs; also, of drugs: slightly intoxicating and tending to produce such effects.
6. (chiefly African-American Vernacular, slang) Pleasing in some way; excellent, fantastic, great.

mellow n

1. The property of being mellow; mellowness.
2. (specifically) A comfortable or relaxed mood.
3. (African-American Vernacular) Also main mellow: a close friend or lover.

mellow v

1. (transitive)
   1. To cause (fruit) to become soft or tender, specifically by ripening.
   2. To cause (food or drink, for example, cheese or wine, or its flavour) to become matured and smooth, and not acidic, harsh, or sharp.
   3. (archaic except British, regional) To soften (land or soil) and make it suitable for planting in.
   4. (figuratively)
      1. To reduce or remove the harshness or roughness from (something); to soften, to subdue, to tone down.
      2. To cause (a person) to become calmer, gentler, and more understanding, particularly from age or experience.
      3. (chiefly passive voice) To cause (a person) to become slightly or pleasantly drunk or intoxicated.
      4. (also reflexive, originally US, informal) Followed by out: to relax (a person); in particular, to cause (a person) to become pleasantly high or stoned by taking drugs.
2. (intransitive)
   1. Of food or drink, or its flavour: to mature and lose its harshness or sharpness.
   2. (archaic except British, regional) Of soil: to be rendered soft and suitable for planting in.
   3. (figuratively)
      1. To lose harshness; to become gentler, subdued, or toned down.
      2. (originally US, informal) Followed by out: of a person: to relax; in particular, to become pleasantly high or stoned by taking drugs.