Ruling of saying the word “wayl” (woe)

“Wayl” (woe) is a word used to warn, scare and
threaten people. Seebawayh said: “Wayh” is a word of warning used to
alert a person who is about to fall into a dangerous situation, and “wayl”
is used for the person who has already fallen into it. 

Fath al-Baari, 1/207 

Al-Qurtubi (may Allah have mercy on him) said:

The meaning (of the word wayl) is humiliation, punishment and
doom.

End quote from al-Jaami‘ li Ahkaam al-Qur’aan, 20/181

Al-Khaazin said: Wayl is a word that the Arabs say to anyone
who falls into deep trouble. Its original meaning is punishment and doom.

End quote from Tuhfat al-Ahwadhi, 9/39 

Al-Sa‘di (may Allah have mercy on him) said:

Wayl is a comprehensive word that includes all kinds of
punishment, grief, torment and fear.

Tafseer al-Sa‘di, p. 813 

If a person uses this word deliberately to threaten and warn
someone against doing an action that is contrary to sharee‘ah, there is
nothing wrong with that. For example, the Prophet (blessings and peace of
Allah be upon him) said: “Woe (wayl) to the heels from the Fire! Do wudoo’
properly.” Narrated by al-Bukhaari, 96; Muslim, 241. 

If a person uses this word for everything that he thinks is
different to what he thinks, even if the issue is a matter of controversy
among the scholars, or he uses it for something that is contrary to a
Sunnah, such as saying, “Woe (wayl) to the one who does not recite Soorat
al-Faatihah behind the imam” or “Woe (wayl) to the one who does not offer
the Sunnah prayer of Maghrib,” this is not permissible. 

If a person says it deliberately, praying against himself,
that is not permissible, unless he says it in a way that is connected to a
condition, such as if he says, “Woe to me (ya wayli) from the punishment of
Allah if He does not forgive me,” as ‘Umar (may Allah be pleased with him)
said when he was stabbed: “Woe to me and woe to my mother if I am not
forgiven.”

Narrated by Imam Ahmad in al-Zuhd, p. 125 

With regard to the view of the one who says that it erases
the good deeds that came before it, this is not correct because there is no
evidence for that. 

If a person says it deliberately or otherwise in an
inappropriate manner in which it is not permissible to say it, he has to ask
Allah for forgiveness and not do it again. 

For more information please see the answer to question no.
81874 and
128040 

And Allah knows best.