jmdict 1184860 Active (id#2229178)
下弦の月
【 かげんのつき 】
  1. [exp,n]
    • last quarter moon
    • third quarter moon
    • waning half-moon
    Cross references:
    • ⇔ see: 2147030 上弦の月 1. first quarter moon; waxing half-moon
    • ⇐ see: 1263090 弦月【げんげつ】 1. half-moon
History:
    • A 2023-04-21 19:44:29 Robin Scott (id#2229178)
    • Diff
      @@ -12,0 +13 @@
+<xref type="see" seq="2147030">上弦の月</xref>
    • A 2023-04-21 02:04:10 Jim Breen (id#2229134)
    • Diff
      @@ -10,0 +11 @@
+<pos>&exp;</pos>
    • A 2023-04-21 02:03:46 Jim Breen (id#2229133)
    • Diff
      @@ -14 +14 @@
-<gloss>waning half moon</gloss>
+<gloss>waning half-moon</gloss>
    • A* 2023-04-20 18:54:15 Brian Krznarich (id#2229095)
    • Comments
      harmonize with 下弦. See comments on 下弦, and eventually on 弦月.

This cannot be "waning moon".  A waning moon is the entire period between the full moon and the next new moon. 下弦 is a half moon (and even in the best case, as a bow with a string that you could maybe imagine as being a less-than-half-moon, it couldn't possibly exceed 1/2). 

This is a very old "bad" gloss, and has been the source of a lot of (I would argue) bad example sentences in the wild.

https://ejje.weblio.jp/content/waning+moon
This seems closer to "waning moon" 欠けた月

Image search will give you results with all kinds of percentages of moon visible, the restriction being that it is getting darker.  

Search for 下弦の月 and you'll get half-moons.
    • Refs
      https://www.dictionary.com/browse/waning-moon
the moon at any time after full moon and before new moon (so called because its illuminated area is decreasing).
    • Diff
      @@ -12 +12,3 @@
-<gloss>waning moon</gloss>
+<gloss>last quarter moon</gloss>
+<gloss>third quarter moon</gloss>
+<gloss>waning half moon</gloss>
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