GG5: a coward; a dastard; a sneak; a poltroon; 《口》 a chicken.
Reverso

Poltroon? Gadzooks!
Yes, I'd avoid those dated words.

@@ -16 +16,4 @@
-<gloss>coward (esp. of a liar, a cheat, etc.)</gloss>
+<gloss>coward</gloss>
+<gloss>sneak</gloss>
+<gloss>chicken</gloss>
+<gloss>yellow-belly</gloss>

I don't think the suggested amendment would be an improvement.

dastard
/ˈdɑːstəd/
noun DATED•HUMOROUS
a dishonourable or despicable man.

https://tr-ex.me/翻訳/日本語-英語/卑怯

臆病者	143864	63.8%
卑怯者	81711	36.2%

臆病	442829	49.0%
卑怯	461165	51.0%

https://popnmusic.fandom.com/wiki/Sumire
You dirty coward! (卑怯者めっ! Hikyousha me!?)

https://touhou.fandom.com/wiki/Fairy_Wars:_Route_B
そうよ！
卑怯者め！
Right!
You dirty coward!

Characteristics of a 卑怯な人　（臆病 is just one characteristic）
https://domani.shogakukan.co.jp/587534/2#i-2
平気で嘘をつく
責任転嫁が上手い

卑怯者	81711	98.8%
ひきょう者	1021	1.2%

The English dictionary backed up my intuition for once, saying "dastard" is [dated].

I was going to separate [1] and [2] following 卑怯, but sankoku puts them under one sense, and googling  卑怯者 臆病者 違い suggests that 臆病 is about weakness and timidity(弱虫), while this term is about underhandedness and lack of morals or honor.

There really is only one gloss here.  I could say "a cheat", "a dastardly person", "an underhanded person"... These are not practical glosses, they are attempts at a definition.  The politician is a coward.  Get back here you coward! I ran away, I abandoned my men, I was a coward...  Not "I was a dastardly person". 

Image search for 卑怯者 returns some nice manga stills.

Reverso examples are all "coward", including a "yellow bellied coward", and 
卑怯者ではない  I am a man of honor

(Just notice Paul from 2011 on submit. Yes, dastard seems too obscure, and I was initially just going to change to "dastardly person").

@@ -8,0 +9 @@
+<ke_inf>&sK;</ke_inf>
@@ -15,2 +16 @@
-<gloss>coward</gloss>
-<gloss>dastard</gloss>
+<gloss>coward (esp. of a liar, a cheat, etc.)</gloss>

daijr
http://eow.alc.co.jp/ひきょう/UTF-8/?ref=wl
Google ("ひきょう者")

If dastard seems too obscure, "dastardly person" would be fine, too. Some version of dastard/dastardly sums this up better than anything else I can think of.

@@ -6,0 +6,3 @@
+</k_ele>
+<k_ele>
+<keb>ひきょう者</keb>
@@ -13,0 +16,1 @@
+<gloss>dastard</gloss>