Not to worship idols is one of the 10 commandments; according to a quick Google search "finger crossed" originates from either the Christian cross or other superstitious beliefs.
Does this mean that using the term "fingers crossed" is forbidden for Jews?
B"H
As per a comment, there is a discussion on yeshiva.org that seems to be discussing the same expression in Hebrew, although it's not entirely clear:
Question:
Respected Rabbi,
I understand that the expression "hold thumbs" (מחזיק אצבעות) has its origins in Christianity, where the gesture creates a kind of cross.
Is it therefore forbidden to use the expression as it is commonly used today, and why?
Thank you very much.
Answer:
To the questioner, peace!
Regarding the origin of the expression, I have heard various opinions. In any case, even an expression that has an idolatrous origin is permitted to be used when it does not have the same meaning for us. A simple example of this is the use of the name of the month Tammuz (such as in Mishnah Taanit 4:5-6), even though its origin is idolatrous (see Ezekiel 8:14 and Daat Mikra commentary there, and in Moreh Nevuchim 3:29, which connects the name of the month to the name of the idol; see also Schwartz's edition there, note 47). It is possible, however, that there is a reason to avoid doing so (and see Tzitz Eliezer 8:8:8 and 9:14:4, which states that it is preferable to avoid mentioning the names of the Christian calendar months, which were named after idols). In general, it is preferable to avoid using vague expressions whose literal meaning is not given much thought. However, it is possible to intend to give these words a positive meaning: not "I hold my fingers as a sign of your success" but "I hold your fingers," like the original Hebrew expression for support - "hold your hand" (Zechariah 8:9; Ezra 1:6; Sheviit 5:9) or "hold your right hand" (Isaiah 41:13; 45:1).
If it is indeed discussing the expression, it would seem that it would be permitted to use the expression in a general way, without specifying that one is doing so as a form of hoping for luck etc., but just as some kind of moral support, like "I've got your back".
