To encroach on a fellow's trade/ to trespass upon someone's occupation/ to compete with
To compete in a friend's business in an unfair way. To work in the same profession that someone else was already practicing and thereby harm that person's economic standing.
("And he did not defile his neighbor's wife"—meaning that he did not encroach on his fellow's trade, and "he did not come near a menstruant woman"—meaning that he did not benefit from charity funds"—Sanhedrin 81a; and also: "One who encroaches on his fellow's livelihood is as though he had come upon his wife"—R. Nachman of Breslov / Likkutei Moharan, Torah 23, 3.)
Translation
Expression (Hebrew)
יָרַד לְאָומָּנוּת חֲבֵרוֹ
Definition (Hebrew)
הִתְחָרָה בָּעֵסֶק שֶׁל חֲבֵרוֹ בְּצוּרָה לֹא הוֹגֶנֶת. עָסַק בְּאוֹתוֹ מִקְצוֹעַ שֶׁבּוֹ עָסַק אָדָם אַחֵר קֹודֶם לָכֵן וּפָגַע בְּמַעֲמָדוֹ הַכַּלְכָּלִי.
("וְאֶת אֵשֶׁת רֵעֵהוּ לֹא טִמֵּא - שֶׁלֹּא יָרַד לְאֻמָּנוּת חֲבֵרוֹ, וְאֶל אִשָּׁה נִדָּה לֹא קֵרַב- שֶׁלֹּא נֶהֱנָה מִקֻּפָּה שֶׁל צְדָקָה"- סַנְהֶדְרִין פא ע"א, וְכֵן:"הַיּוֹרֵד לְפַרְנָסַת חֲבֵרוֹ כְּאִלּוּ בָּא עַל אִשְׁתּוֹ"- ר' נַחְמָן מִבְּרֶסְלָב/ לִקּוּטֵי מוהר"ן, תּוֹרָה כ"ג, ג')