A member of the congregation has just chanted a four-page haftarah and kept everyone riveted. Someone calls out, “Yasher Koah!” and a bunch of other congregants echo the sentiment. What are they saying?
The literal meaning is something akin to “straight strength.” Koah (כּוֹחַ) means “strength” and yishar (יִישַּׁר) (the dictionary puts “yasher” (“יְיַשֵּׁר”) into parentheses) means “may it be straightened.” Colloquially, the expression means “right on!” or “looking forward to your being strong enough to do this kind of thing again.”
Now, then. Let’s talk about some grammar. Your correspondent knows virtually nothing about Hebrew grammar, but every little bit she learns is a delight. If we wipe away the years of colloquializing and Yiddishizing (which has even yielded “shkoyach” in some regions), we end up with these properly pronounced Hebrew expressions of accolade:
Some say that the term originates from a Talmudic commentary on Moses smashing the Tablets, and then God approved of the action, as it happened before he would have had to punish the people under that law. (Shabbat 87a.) They base this on the words “asher shibarta,” the tablets “which you broke.” They take it as a play on words with “yishar kohekha.”
It also is said that in olden times the Torah reader had to hold the scroll vertical to read from it, so that those around him could see. That required “straight strength” not to drop it. Which is essentially the very thing that Moses did, drop the Law.
The proper response to a man upon receiving this encouragement is “Barukh tihyeh,” or to a woman “Berukhah tihyi,” “May you be blessed.”