gg5, prog, wisdom
打ち解け	169,093	90.3%	
うち解け	15,650	8.4%	
打解け	2,434	1.3%

@@ -10,0 +11 @@
+<ke_inf>&sK;</ke_inf>
@@ -13,0 +15 @@
+<ke_inf>&sK;</ke_inf>
@@ -24,2 +26,5 @@
-<gloss>to feel comfortable (with someone)</gloss>
-<gloss>to be unreserved</gloss>
+<gloss>to become unreserved</gloss>
+<gloss>to lower one's guard</gloss>
+<gloss>to become friendly (with)</gloss>
+<gloss>to feel at ease</gloss>
+<gloss>to take a relaxed attitude (to)</gloss>

Was useful still in 1828, I guess
https://webstersdictionary1828.com/Dictionary/declare
To declare one's self, to throw off reserve and avow one's opinion; to show openly what one thinks, or which side he espouses.

https://eow.alc.co.jp/search?q=打ち解ける
打ち解ける
come [get, break, crawl] out of one's shell〈話〉（内気だった人が）

打ち解けるためのゲーム
ice-breaking game（初対面の人たちが）

（人）に打ち解ける
open up to

お互いに打ち解ける
feel comfortable with one another
open up to one another

https://ejje.weblio.jp/content/打ち解ける
研究社 新和英中辞典での「打ち解ける」の英訳
うちとける 打ち解ける
be frank [open, candid] 《with somebody》

打ち解けて話す
have a friendly chat [heart‐to‐heart talk] 《with》
talk freely 《to》

打ち解けた調子で
in a chatty [familiar] tone
打ち解けない
be uncommunicative
be reserved
《口語》 be standoffish.

Interesting. Was juxtaposed with 心を開く (which I just edited as well), same meaning, mixing up vocab.

Same stubborn old woman, after trying to win her over:
このばあさん、全然心開かねえぞ。　1. "This old woman isn't opening her heart at all" 2."This old woman just refuses to open up"
1: If Himmel or Heiter were here, ... "半一くらいで打ち解けているはずなのに" "she would have opened up in half a day".
2: "Sorry, Himmel or Heiter could have gotten her to open up in half a day."

 
"to throw off reserve" appears to be quite well dated.  First non-dictionary result on google was a text from the mid-1800s.

1 result on all of reddit is a false-positive. no news results, no twitter results.

"frank" seems to be pointed at a no-longer-common connotation of the word.  I can be frank with someone who I don't like at all, and am totally uncomfortable being around(frank = direct-and-to-the-point. "let me be frank"). The weblio ref puts "frank" next to "open" (which I've added) and "candid" which is better, but also has connotation issues.  I think "unreserved", which hints at   打ち解けない = reserved, works better.

@@ -23,3 +23,3 @@
-<gloss>to open one's heart</gloss>
-<gloss>to throw off reserve</gloss>
-<gloss>to be frank</gloss>
+<gloss>to open up (to someone)</gloss>
+<gloss>to feel comfortable (with someone)</gloss>
+<gloss>to be unreserved</gloss>

EDR日英対訳辞書
20k hits

@@ -8,0 +8,3 @@
+</k_ele>
+<k_ele>
+<keb>うち解ける</keb>