Tachlis (תכלית) is a Yiddish word that comes from Hebrew, where it means “end,” “entirety,” “ultimate” or “purpose.”
King David uses this word in Psalms, where he describes his feelings toward evildoers as “tachlis of hatred” (תכלית שנאה שנאתים).1 In this context, it means “ultimate.”
A classic use of this word is the Hebrew adage, “The tachlis of knowledge is to know that we cannot know You” (תכלית הידיעה שנדע שלא נדעך).2 In this case, the word quite clearly means “purpose.”
An example in Kabbalistic literature is the description of G‑d’s infinite light descending “down to no tachlis” (למטה עד אין תכלית).3 In this case, it is best interpreted as “end.”
One famous Chassidic use of tachlis is in Tanya, where we read that “the tachlis of humankind… and all the worlds … is that G‑d should have such a dwelling place here below.”4
In common Yiddish parlance, the word tachlis can also refer to real-world action, working to earn a living, practicality, or usefulness.
“Talking tachlis” can be a no-holds-barred conversation about life’s major decisions such as settling down with a profession, a spouse etc., or identifying purposeful goals in life.
My mom often said tack less. I always thought it meant without craft. For example, a tack and I'm not sure if I am spelling that correctly would be a crafty way of sailing into the wind. Tach less would be sailing with the wind, which is the fastest or I guess you could say the ultimate or fastest way to sail. I did have sailing lessons. So, basically tach less would be similar to tact less, and having no tact is the ultimate or most honest way to be.
Thanks to your reply, which I found quite amusing, I doubt I will ever forget the original word or your interpretation ever again!